Current sensors for overload trip and other purposes have been known heretofore. For example, circuit breakers and fuses have been used to monitor the current in an electrical circuit and to protect the circuit from a current overload. The means that these devices have used to sense the actual current flow have been magnetic, thermal (such as bimetal, fuses, etc.) and electronic. Whatever the means that have been used, the circuit current has been sensed and an appropriate signal has been provided when an overload occurs. In the case of circuit breakers, what has been primarily used has been magnetic sensing and/or thermal current sensing, the thermal current sensing being accomplished primarily with bimetal or heated wire expansion techniques. In the case of fuses, the heated one-time link has been the method for monitoring and controlling current overloads. However, detecting and replacing a "blown" fuse presents an undesirable maintenance problem.
While these prior current sensing devices have been useful for their intended purposes, they have been handicapped in a variety of ways. For example, disadvantages of such prior current sensing devices include lack of sensitivity due to the requirement for a mechanical output, this being a particular problem for low current devices. Also, such prior devices have had wide tolerance envelopes, susceptibility to environmental effects, long term stability problems due to wear and drift, compatibility problems with electronic control circuits, manufacturing and calibration problems, response characteristics fixed by their mechanical configurations which made them difficult to modify for particular needs, coordination problems, and time delay difficulties of the magnetic devices. Potential difficulties with various electronic current sensing devices included input isolation and protection, power supply requirement, desired response under all conditions, indirect drive to open contacts, ultimate fusibility, reliability at extreme temperatures (beyond design limits), cost, unpredictable factors with respect to new technology, and radiation hardness.